The progress of this Program-Project is reported under three major headings: 1. The cell surface in the suppression and stimulaton of cell division (Mazia). The extreme case of the fertilization of the sea urchin egg, which rapidly transforms an inert unfertilized egg to an actively synthesizing and dividing zygote has been investigated. A component of the outer cell surface has been implicated in the release of DNA synthesis and cell division at fertilization. Related studies deal with the membrane changes at the fusion of egg and sperm cell and the chemical properties of chromatin between the time of fertilization and the time of the first mitosis. 2. RNA and DNA in early development. With a newly developed technique, accurate data on the rate of synthesis and the half-life of RNA in sea urchin eggs and in HeLa cells have been obtained. The polyadenylated messenger RNA in the cytoplasm of sea urchin eggs has been studied and it has been shown that fertilization is followed by an increase in the chain length of polyadenylated mRNA. Pyrimidine isostichs (tracts in the DNA which are short polymers of thymidine) have been isolated from both the redundant and unique DNA components of the chromatin of sea urchin embryos. 3. Molecular biology of the differentiation of muscle. Following the first achievement of the synthesis of myosin in an in vitro system, the technical aspects have been pursued with the aim of improving the methods; such improvements are required in order to define completely the sequence of events between gene transcription and the final production of myosin. The mRNA for myosin in muscle has been shown to be different from the mRNA for myosin in other cells, a problem of major importance now that it is known that myosin is involved in many kinds of cell motility. Studies of the changes of the cell surface during the differentiation of muscle continue; there is solid evidence that the myogenic cells have a surface antigen not present on other cell types in the chick embryo.